1. Field of the Invention
The present invention relates to a plasma processing apparatus and, more specifically, to a plasma processing apparatus suitable for etching, sputtering for depositing a thin layer of metal on a substrate, cleaning a work, ashing, and forming a film on a substrate.
2. Description of the Prior Art
The plasma process is a process in which highly active radicals and ions produced by ionizing a specific material are applied to a work to etch the work, to deposit a film on the work, to deposit a layer of metal on the work by sputtering, to clean the work or to ash the work, and the plasma processing apparatus is used for carrying out such a process.
The conventional plasma processing apparatus comprises a vacuum vessel having a source gas inlet opening and a discharge opening and defining a plasma processing chamber, and a device for generating electromagnetic waves or the like for supplying energy necessary for producing the plasma of the source gas.
The plasma process uses the high energy of the radicals and ions and it is therefore a feature of the plasma process that the plasma process is applicable to various desired processes including etching and film deposition by selectively deciding processing conditions, such as the respective densities of radicals and ions, and the temperature of the work. Efficient production of radicals and ions is essential to the plasma process.
A high-frequency electromagnetic wave of a frequency on the order of 13.56 MHz has been used as an energy medium for producing a plasma. Recently, it has been found that a high-density plasma can be produced by using a microwave of a frequency on the order of 2.45 GHz, and plasma processes employing microwaves have become the object of attention. Several apparatuses for such plasma processes have been proposed.
There have been proposed, for example, a method and an apparatus based on a plasma CVD process using a microwave (hereinafter, referred to as "MW-PCVD process"), for forming an amorphous silicon (hereinafter, referred to as "a-Si") film for optical elements and electronic elements, such as semiconductor devices, electrophotographic sensitized devices, image pickup sensors, image pickup devices and photovoltaic devices.
This MW-PCVD process uses a high-density plasma produced by ionizing neutral molecules by electrons efficiently accelerated by an electric field established by a microwave, and a magnetic field created by a magnetic field creating device disposed outside the discharge chamber. Particularly, determination of the magnetic field intensity so that the cyclotron frequency, namely, a frequency at which electrons traverse, coincides with the frequency of the microwave enables efficient production of a plasma. When the frequency of the microwave is 2.45 GHz, which is used generally, an optimum magnetic field intensity is 875 G (gauss).
The MW-PCVD process is advantageous over the high-frequency plasma process in that the discharge pressure is in the wide range of 10.sup.-4 to 10 torr, and hence, the mean free path of ions is greater than the width of the ion sheath under a low pressure in the range of 10.sup.-4 to 10.sup.-3 torr. Accordingly, when the MW-PCVD process is applied, for example, to an etching apparatus, perpendicular etching is possible because ions fall perpendicularly on the surface of a work. Furthermore, in the MW-PCVD process a large quantity of excited gas can be produced under a pressure in the range of 0.1 to 10 torr, and a work can be processed without being damaged, because the energy of the incident ions is as low as 20 eV.
However, since a microwave is introduced into the discharge chamber through a microwave transmission window, which is small as compared with the sectional area of the discharge chamber, in the conventional plasma processing apparatus, the microwave is absorbed by a plasma prevailing around the microwave transmission window after the plasma has been produced, and hence, it is impossible to produce the plasma uniformly within the discharge chamber.
Incidentally, Japanese Patent Laid-open (Kokai) No. 60-120525 discloses a microwave plasma processing apparatus as shown in FIG. 12. In FIG. 12 there are shown a discharge chamber 1201, a processing chamber 1202, a microwave transmission window 1203, a rectangular waveguide 1204, a plasma current 1205, a plasma transmission window 1206, a work 1207, a work holder 1208, a work table 1209, an evacuating system 1210, a solenoid 1211, a magnetic shield 1212, a first gas supply system 1213, a second gas supply system 1214, and cooling water supply and discharge lines 1215.
In producing a plasma in this microwave plasma processing apparatus, the discharge chamber 1201 and the processing chamber 1202 are evacuated in a high vacuum by the evacuating system 1210, gases or a gas is introduced into the discharge chamber 1201 by the first gas supply system 1213 and/or the second gas supply system 1214 so that the gas pressure in the discharge chamber 1201 is in the range of 10.sup.-6 to 1 torr. A microwave generated by a microwave source, not shown, is introduced through the rectangular waveguide 1204 and the microwave transmission window 1203 into the discharge chamber 1201 and, at the same time, a magnetic field meeting conditions for electron cyclotron resonance is applied at least to a part of the discharge chamber by the solenoid surrounding the discharge chamber 1201. When the microwave source is a magnetron of 2.45 GHz, a magnetic flux density meeting the condition for electron cyclotron resonance is 875 G. The discharge chamber 1201 is constructed so as to meet conditions for a microwave cavity resonator to enhance discharge efficiency. For example, for a cylindrical cavity resonance mode of TE.sub.113, the discharge chamber 1201 is 17 cm in inside diameter and 20 cm in inside height. To meet conditions for the resonance mode, it is preferable that the microwave transmission window has a comparatively small size. In the conventional microwave plasma processing apparatus, the size of the microwave transmission window is the same as that of the inside section of the waveguide for microwave, ordinarily, a waveguide of 109.22 mm.times.54.61 mm in inside size (JIS WRJ-2). A plasma produced in the discharge chamber flows through the plasma inlet window 1206 toward the work 1207. After the plasma has been produced, the microwave introduced into the discharge chamber is absorbed by the plasma in the vicinity of the microwave transmission window 1203. This tendency increases as the plasma density increases reducing the propagation of the microwave within the discharge chamber; consequently, it is impossible to produce the plasma uniformly in the discharge chamber, and hence, it is impossible to process the work uniformly. To process the work uniformly by avoiding such a problem, the plasma inlet window 1206 is formed small in size, which entails problems such that the plasma is applied only to a limited area on the surface of the work and that the plasma produced in the discharge chamber cannot be used effectively.
On the other hand, in the fields of microwave communication and radar, which are entirely different from the field relevant to the present invention, an antenna having a flat plate provided with a slot or slit has been developed. Such an antenna is disclosed in F. J. Goebels and K. C. Kelly, IRE Transactions on Antenna and Propagation, AP-9, p. 342, July, 1961. Recently, the development and improvement of antennas have been made for receiving satellite broadcasts and studies relating to such activities are disclosed in N. Goto and M. Yamamoto, Denshi Tsushin Gakkai Gijutsu Kenkyu Hokoku A, pp. 57-80 and p. 43, 1980, and H. Sasazawa, Y. Oshima, H. Sakurai, S. Ando and N. Goto, Denshi Tsushin Gakkai Gijutsu Kenkyu Hokoku A, pp. 86-142 and p. 43, 1986.
However, microwave communication and radars are the principal purposes of those slotted flat plate antennas, and those slotted flat plate antennas are not at all intended for application to producing a plasma.